In a recent electronic engine control system for a vehicle, for example, as disclosed in Japanese Patent Publication No. H11-22515, an engine torque (a target torque) demanded by a vehicle driver is calculated based on, for example, a stroke of an acceleration pedal operated by the driver, a target opening degree of a throttle valve is calculated from the target torque, and an actual opening degree of the throttle valve is controlled to be at the target opening degree, so that a drivability having a higher response to the acceleration pedal operation by the driver is realized.
In the above prior art (Japanese Patent Publication No. H11-22515) for the throttle control, the target torque is corrected so that the target opening degree of the throttle valve is calculated based on the corrected target torque, in view of useless time of the electronic throttle control system, a delay of a response for operating the throttle valve, a delay of an air flow for intake air in an intake air pipe, and so on.
As above, according to the above electronic throttle control system, the target torque is corrected to calculate the target opening degree of the throttle valve, in consideration of the useless time and the delay of the response of the system, as well as the delay of the air flow. However, according to a recent electronic throttle control system, a motion restriction is added to the control for the throttle valve, further in view of exhaust gas emission. As a result, an actual opening degree of the throttle valve (i.e. an intake air amount actually charged into an engine cylinder) is restricted by such motion restriction, when the target torque (a target intake air amount) is drastically changed in a stepwise manner. Then, the intake air amount is deviated from the target amount, resulting in deterioration of the response for the throttle control.
According to another prior art, for example, as disclosed in Japanese Publication No. 2002-201998, it has an object to improve control accuracy for an air-fuel ratio during a transition period. For that purpose, a delay is given to the target opening degree of the throttle valve. The opening degree of the throttle valve at a valve closing timing of an intake valve is estimated, based on the target opening degree of the throttle valve (before the delay is given) and a characteristic for a delay in response of the electronic throttle system. An intake air amount is calculated based on the above estimated opening degree of the throttle valve. And a fuel injection amount is calculated from the estimated intake air amount.
When it is compensated, in the above prior art (JP Publication No. 2002-201998), to reduce a deviation between the target value and the actual value, the compensation is carried out for not only the deviation but also the delay for the target opening degree of the throttle valve. A compensation gain may become too large and an overshoot for the opening degree of the throttle valve may occur. As a result, stability for the throttle control may be deteriorated.